Create specific gas clustering for market model

[ulfmueller]

Here the market model gets a one-node per market region resolution for the electrical side. S.th. similar would be nice to have for the gas sector and the corresponding non-electrical sectors such as heat.
Maybe as a starting point, similar to what there is for electricity we create an analogy for the gas sector - i.e. it will be clustered to the same resolution. The attached components should not be aggregated (see current electrical solution), so the interface to the grid optimization remains to be trivial.